1. Field of the Invention:
This invention relates to a pneumatic boost device, particularly for control of a clutch of a motor vehicle.
2. Discussion of the Background:
The most widely used control of clutches, especially heavy-duty ones, is position control by the transfer of a volume of oil, with a pneumatic boost. To avoid the drawbacks of oil transfer systems, pneumatic and mechanical positioning systems have been proposed and developed like the one illustrated in FIG. 1. The system there shown is a follower system with an input requiring a small control energy and an output with a much greater available energy. The displacement at the output, except for the initial clearance, is perfectly linear as a function of the input set point.
The device consists of a body in which is placed a cylinder 1 with a pneumatic piston 2, which displaces an actuating rod 3 connected to the clutch mechanism. A pusher 4 is fed compressed air via orifices 5 and 6. A check valve 7 controls the intake of the compressed air into the (top) chamber of cylinder 1. A spring keeps this intake check valve 7 closed in the normal situation, the sealing being made on a seat 9, so that there is no boost. The chamber of cylinder 1 is connected to the atmosphere by an orifice 10 because a clearance exists between piston 2 and seat 8 of the check valve. The manual displacement of pusher 4 by rod 25 closes the exhaust of check valve 7 at seat 8 and opens the intake by seat 9 so that boost is applied to the chamber of cylinder 1.
The defect of this type of system is, paradoxically, that it is too reliable, i.e., it follows the displacement of the input set point too linearly. The characteristic of the gears usually used in a transmission is to require a large clutch opening clearance. With a linear control, this clearance is felt at the pedal and reduces the travel of the pedal available for metering the torque and so reduces the gradualness of the control.